Background and Aims: A prospective double-blinded study with preset sonographic criteria has not been performed to assess the accuracy of duplex ultrasonography in determining the patency of transjugular intrahepatic portosystemic shunts (TIPS). The purpose of this study was to determine the sensitivity and specificity of duplex ultrasonography in predicting shunt malfunction using accepted preset sonographic criteria.

Methods: Sixty ultrasonographic and venographic follow-up comparisons were made on 38 cirrhotic patients who had undergone TIPS placement for variceal bleeding (n = 28) or intractable ascites (n = 10). Ultrasonographic results were analyzed by one of two board-certified ultrasonographers without knowledge of venographic findings.

Results: Of the 31 occluded (n = 8) and stenotic (n = 23) shunts, ultrasonography accurately predicted a shunt malfunction (occlusion or stenosis) in only 11 studies and incorrectly predicted patency in 20. Compared with venography, ultrasonography had a sensitivity of 35% and a specificity of 83% in predicting TIPS stenosis or occlusion.

Conclusions: These results suggest that duplex sonography is not a sensitive test in predicting the presence of a hemodynamically significant stenosis and that shunt status should be assessed by venography and direct portal pressure measurements until a more reliable and proven noninvasive ultrasonographic criterion is devised.

COMMENTS

After almost a decade of clinical investigation, the transjugular intrahepatic portosystemic shunt (TIPS) is now widely accepted as a treatment for complications of portal hypertension. For most patients, TIPS is successful initially, but over time shunts become stenotic or occluded. Shunt malfunction can be significantly reduced by routine surveillance; indeed, with proper surveillance and percutaneous intervention when necessary, TIPS are maintained successfully for extended periods of time.

The best method for TIPS surveillance is unknown. Strategies for surveillance have varied widely among investigators, with sonography, shunt venography, and endoscopy being used alone or in combination at many different intervals. A common tactic is for a baseline study to be performed within the first few days of shunt placement; additional evaluations are often performed at 1, 3, and 6 months, and every 6 months thereafter. Among all the available approaches to TIPS surveillance, venography at close intervals would seem the most effective for maintaining TIPS patency. Venographic evaluation allows the direct measurement of portal pressure, as well as the anatomic demonstration of varices. Moreover, venography permits shunt revision when necessary during the same procedure. Sonography provides only indirect assessment of shunt function and does not allow for revision; endoscopy is accurate for diagnosing varices, but is invasive and cannot be used to revise a malfunctioning shunt.

TIPS venography can be performed in the outpatient setting with minimal risk. However, despite its advantages, venography is invasive and inconvenient for repeated surveillance examinations. Most patients would prefer a less invasive means of shunt surveillance if shunt function could be acceptably monitored. Sonography offers a noninvasive alternative to venographybut is it adequate for TIPS surveillance? This is the question raised by Owens et al. in the study highlighted above. Owens et al. conducted a prospective double-blind study to assess accuracy of sonography in patients undergoing TIPS surveillance. They found that, compared with venography, sonography is inaccurate and unreliable. They conclude that routine TIPS surveillance is best performed by venography with direct portal pressure measurements.

To put these results into perspective, it is important to note that the quality of any imaging modality for TIPS surveillance depends on the endpoint used to judge shunt patency. The most important clinical endpoint in any evaluation of TIPS surveillance is rebleeding from portal hypertension. To date, controlled trials comparing different modes of TIPS surveillance have not used rebleeding as an endpoint; however, the efficacy of individual surveillance methods can be inferred from long-term follow-up studies of TIPS. Interestingly, Rossle et al.,1 who have the lowest 1-year rebleeding rate of any reported series (15% rebleeding at 1 year) used only ultrasound for routine TIPS surveillance. By comparison, a somewhat higher rebleeding rate (20% at 1 year) was recently reported by the group from the University of Oregon,2 where angiography was performed at 6-month intervals for routine surveillance. Although one might argue that rebleeding is dependent not only on TIPS surveillance but also on other factors such as patient demographics and reintervention methods, the low rebleeding rate achieved by Rossle et al. suggests that ultrasound is an effective surveillance modality in their hands.

Another means of assessing the value of a given mode of TIPS surveillance is to compare it with a gold standard such as venography. This is the strategy used by Owens et al, who compared sonography with venography and calculated a sensitivity and specificity for sonographic detection of shunt abnormalities. The goal of such a comparison is clear; however, the comparison itself can be quite complex due to the variety of measurements that are available to judge shunt patency. First, a number of parameters have been used to evaluate shunt function by Doppler ultrasound. These include (1) identification of flow within the shunt; (2) determination of maximal peak flow velocity (MPFV) within the shunt (or within the proximal, mid, or distal segments of the shunt); (3) identification of the direction of flow within the intrahepatic portal vein branches; (4) determination of peak flow velocity in the main portal vein; and (5) documentation of the direction of flow in the draining hepatic vein. In most studies, a threshold value is determined for each velocity parameter and/or a comparison is made with the results of a prior examination. Many of these parameters, particularly the velocity calculation, can vary with the equipment used and are operator-dependent. For example, the MPFV measured in a given patient is dependent on the type of ultrasound machine used and its calibration, as well as the skill and technique of the sonographer. Second, the venographic findings to which ultrasound is compared vary among investigators. Shunt morphology (>50% diameter stenosis), shunt function (portosystemic gradient [PSG] 15 mm Hg), and clinical symptoms either alone or in combination have been used in published series comparing ultrasound with venography in TIPS follow-up.

In the study by Owens et al., the venographic definition of stenosis was a greater than 50% stenosis and/or a PSG 15 mm Hg. The ultrasound definition of stenosis included a maximal peak flow velocity of less than 60 cm/sec and/or other criteria to suggest a hemodynamically significant stenosis. The inclusion of this “and/or” criteria makes the ultrasound diagnosis somewhat operator-dependent because a single criterion could be used to call a shunt either normal or abnormal. Using these definitions, Owens et al. report a 35% sensitivity and 83% specificity for the ultrasound detection of shunt dysfunction.

Owens’ results differ from those reported previously for ultrasound detection of TIPS dysfunction. Indeed, initial reports were quite favorable, with 100% sensitivity and over 90% specificity,3-5 prompting one author to write that ultrasound is a “nearly ideal” method of follow-up.4 More recent reports show a downward trend in accuracy for sonography that is perhaps more realistic. We reported a 78% sensitivity and 99% specificity for TIPS sonography, using a MPFV 50 cm/sec for a greater than 50% stenosis on angiography.6 Recently, Haskal et al.7 found a 57% sensitivity and 89% specificity using a MPFV less than 60 cm/sec correlated with PSG 15 mm Hg or return of symptoms.

Are the poor sonographic results reported by Owens et al. just a continuation of this downward trend or is there another explanation? Several aspects of their results are worth noting and may have led to the low ultrasound accuracy. First, the authors report a very high rate of malfunction among the shunts studied. Within the first month, 33% of the shunts were occluded or stenosed, and during the 7-month period of study, 31 occluded or stenotic, shunts were detected in 38 patients. This rate of dysfunction is considerably higher than the rate we have reported.8 Perhaps the high rate of shunt dysfunction in the study by Owens et al. contributed to an increased rate of ascites, which impaired the sonographer’s ability to image the shunts on ultrasound. Second, as noted above, the ultrasound criteria for diagnosing shunt dysfunction were fairly broad, perhaps accentuating variability in interpretation of the ultrasound examinations. In other words, the authors do not report the accuracy of a specific ultrasound finding such as MPFV less than 60 cm/sec, but instead report the accuracy of the entire ultrasound examination, which is a synthesis of the operator’s interpretation of all of the ultrasound findings.

In the end, the study reported by Owens et al. is important because it highlights the potential limitations of ultrasound and emphasizes the need to correlate ultrasound and venography results within a medical center or perhaps for an individual patient. That is, if there is poor correlation between venography and sonography at a given institution or if a shunt cannot be reliably visualized by sonography in a given patient, then venography should be obtained for routine surveillance. On the other hand, we continue to feel that ultrasound is a valuable noninvasive means of following most TIPS patients at our institution. Our data suggest that Doppler sonography is a fairly accurate means of evaluating shunt stenosis and that a good quality, normal post-TIPS ultrasound often obviates the need for routine angiography.